<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <link rel="stylesheet" href="css/triangle.css">
</head>
<body>
    <!--  红色，直径为20px -->
    <div style="width: 20px;height: 20px; border-radius: 10px; background: red"></div>

    <!--  等腰绿色是三角色。90度， 45度;45度-->
    <div class="triangle"></div>
</body>
</html>